March 24, 2008 — If you're tempted to blame your thyroid gland for the numbers on the scale, researchers have some good news and some not-so-good news for you.

In a new study examining thyroid and weight, scientists have found that even a thyroid functioning on the lower end of the normal range is associated with weight gain.

It's long been known that a very overactive thyroid can be associated with weight loss, and a very underactive thyroid linked to weight gain. But the new research suggests that even variations within the normal range of thyroid function are associated with weight changes.

The bad news: It's too soon to know what, if anything, to do about it.

Study Details: Thyroid and Weight

The researchers evaluated the thyroid functioning of more than 2,400 men and women by looking at the results of their thyroid stimulating hormone or TSH test, a blood test to assess thyroid functioning.

"I was struck that small changes within the normal range of TSH were associated with increases in body weight," says Caroline S. Fox, MD, MPH, a study researcher and a medical officer at the National Heart, Lung, and Blood Institute's Framingham Heart Study.

The butterfly-shaped thyroid gland, located in the lower front of the neck, makes thyroid hormone, which in turn goes into the bloodstream and the rest of the body, helping it to use energy, stay warm, and function properly.

The results of the new study, published March 24 in the Archives of Internal Medicine, agree with at least two other recent studies, Fox says.

Fox and her colleagues zeroed in on 2,407 men and women who participated in the famed Framingham Heart Study — Offspring Study, trying to find out if body weights vary within the range of normal TSH values and if changes in the TSH values over a 3.5-year follow-up affect a person's body weight.

A high TSH level reflects an underactive gland; a low TSH usually reflects hyperactivity.

The data were taken from exams done from 1983 to 1987 and 1987 to 1991. The average age at the first data collection was 48.

Results: Thyroid and Weight

"If TSH increased over time, weight was more likely to go up," Fox tells WebMD. They divided the patients into four groups, from those whose TSH increased the least to those whose TSH increased the most. "Women whose TSH changed the most gained 2.3 kilograms [about 5 pounds on average]." Some gained more.

A similar pattern was found in men, she says, although the increases were less. The men whose TSH increased the most gained on average 1.3 kilograms or nearly 3 pounds over the three-and-a-half-year follow-up.

Second Opinion

The study, along with the other two recent reports about the association, provide interesting information, says Roy E. Weiss, MD, PhD, the Rabbi Morris Esformes professor of adult and pediatric endocrinology, diabetes, and metabolism at the University of Chicago Medical Center. He co-authored an editorial to accompany the study.

"What may be within the normal range [of thyroid functioning] for a population may not be the normal thyroid levels for a particular individual," he tells WebMD.

But much more needs to be learned, he says. In short: "It may be your thyroid [associated with your weight gain], but there is nothing we can do about it at this point." It would be premature, he says, to recommend thyroid hormone treatment.

Next Steps: Thyroid and Weight

The findings need to be duplicated in still other studies, Fox agrees. "The Framingham subjects are mostly white. It would be important to see if [this finding] applies to other ethnicities."

Agrees Ramachandran S. Vasan, MD, another co-author of the new study and a professor of medicine at Boston University: "While we show an association, we can't claim cause and effect."

Worth a Trip to the Doc?

While it may be tempting to blame the thyroid when weight increases, weight gain has many causes, Fox says.

"If people feel they have gained an excessive amount of weight they should discuss with their physicians whether thyroid function testing is indicated," she says.